For the filling, boil 200 ml salted water. Stir the couscous into the boiling water, remove the pot from the stove and let the couscous swell in the closed pot for about 10 minutes
Clean, wash and cut the leek into rings. Peel and chop the garlic. Wash lemon hot, grate dry, grate peel thinly, halve fruit and squeeze juice. Wash parsley, shake dry, pluck leaves from the stalks and chop finely
Heat 2 tablespoons of butter, sauté leek and garlic for about 5 minutes. Add lemon juice, zest, couscous and parsley. Stir in egg. Wash the meat, dab dry. Cut meat rind into strips with a sharp knife. With a very long, narrow and sharp knife, cut a pocket from the narrow side along the entire length of the meat
Stuff the filling into the meat pocket. Season meat all around with salt and pepper. Peel onion and chop roughly. Put meat and onion in a flat roasting pan. Braise in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for about 1 hour. Deglaze with white wine after about 30 minutes
Peel the apples and dice them down to the core. Heat 1 tablespoon of butter. Add apple and sugar and fry for about 5 minutes. Deglaze with cider and vinegar. Bring to the boil and simmer for about 10 minutes. Season with cinnamon
Take the roast out of the oven. Increase oven temperature (circulating air: 225 °C/ gas: not possible). Put the roast back into the oven and roast for another 15 minutes